A coaxial resonator filter according to the prior art comprises several coaxial resonators the electromagnetic couplings between which are realised by means of hole and link couplings. FIG. 1 shows a few prior art implementations for realising the couplings. A filter 1 comprises a base plate of a conductive material such as copper, coaxial resonators 3 and an electrically conductive casing 6 which encloses the resonators and includes electrically conductive walls 7 between the resonators. One end (so-called short-circuited end) of each coaxial resonator 3 is attached to the base plate 2 through which it is earthed, and the other end is open, thus constituting a quarter-wave resonator. The walls in the resonator casing may have coupling holes 8 for inter-resonator couplings. The holes are usually located near the short-circuited end of the resonator since the magnetic field and hence the inductive coupling is the strongest there. The size of the hole also affects the strength of the coupling.
The coaxial resonator as such is a resonator type known to a person skilled in the art, comprising a substantially straight inner conductor and an outer conductor coaxially around said inner conductor. The filter according to FIG. 1 has at the upper end of each inner conductor an expansion the function of which is to form a so-called impedance step, or a change of impedance along the longitudinal axis of the resonator. The inner conductors may also be made without said expansion. In FIG. 1, the casing 6 constitutes the outer conductor of each resonator, so it is customary to the call the resonators' inner conductors 3 resonators in short.
In the case depicted in FIG. 1, coupling to a resonator is realised by means of a so-called link coupling. There is beside each resonator a conductive element 4 and 5, which may be a strip, as in FIG. 1, or a wire. The conductive element is conductively attached from a given point to the base plate, being thereby earthed. The strength of the coupling can be determined by adjusting the distance between the strip and the resonator sideways and vertically. This affects the inductive coupling of the resonator. FIG. 1 shows two different ways of realising a link coupling. Strip 5 is a conductive strip shaped like an upside-down U, placed near the resonator. The desired coupling is achieved by shaping the strip and changing its distance from the resonator. The problem in this case has been accurate repeating of the attachment of the strip to the desired location in the manufacturing stage so that the assembly usually requires a lot of working time before the desired characteristics are achieved. It has been noticed that strip 4, which encircles the resonator, can be more easily assembled and repeated than strip 5. However, even this link coupling takes a lot of inspecting and fine-tuning so it is not very well suited to mass production.
Another alternative method of forming the resonator coupling is so-called tapping wherein a conductive strip or wire is brought into contact with the resonator at a given location. The tapping determines the input impedance "seen" by the line to be connected in the direction of the resonator and the correct tapping point can be determined by means of either experimentation or calculation. Since the tapping is fixed, its successful realisation requires that it can be made repeatable with a sufficient accuracy as the strength of the coupling cannot be adjusted after the tapping has been completed.
Use of link couplings and tapping is known from the helix filter technology. For example, FI patent no. 95516 discloses the use of a conductive strip element to produce a link coupling. In addition, said patent describes a link element adjustment that can affect the strength of the coupling. Tapping of a helix resonator is known e.g. from FI patent no. 80542. Helix resonators are usually intended for lower frequencies (say, 450 or 900 MHz) than coaxial resonators, so the layout accuracy is not as critical as in coaxial resonator applications. With higher frequencies, the size of resonator structures gets smaller and thus the required mechanical manufacturing accuracy becomes more demanding.
The problem with the link coupling has been the positioning of the strip. In series production it has not been possible to assemble the strips repeatedly such that the link coupling be identical in all filters, but every filter has to be inspected and adjusted to the desired values by bending the link, usually manually. This increases manufacturing costs and slows down the manufacturing process. Since the aforementioned problems have occurred in conjunction with the link coupling, it has in practice been nearly impossible to implement tapping in the production of coaxial resonator filters in the traditional ways because finding the correct tapping point has been difficult because of the degree of accuracy required in the positioning and soldering.
The use of different couplings (link couplings, tappings, capacitive couplings) as such is kwown in filter technology, but their practical implementations have been in part difficult to realise and manage, especially in coaxial resonator filters.